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<span id="Wisdom-Generic-Export_002fImport-from-Fortran"></span><div class="header">
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<hr>
<span id="Wisdom-Generic-Export_002fImport-from-Fortran-1"></span><h4 class="subsection">7.6.3 Wisdom Generic Export/Import from Fortran</h4>

<p>The most generic wisdom export/import functions allow you to provide
an arbitrary callback function to read/write one character at a time
in any way you want.  However, your callback function must be written
in a special way, using the <code>bind(C)</code> attribute to be passed to a
C interface.
</p>
<span id="index-fftw_005fexport_005fwisdom-1"></span>
<p>In particular, to call the generic wisdom export function
<code>fftw_export_wisdom</code>, you would write a callback subroutine of the form:
</p>
<div class="example">
<pre class="example">  subroutine my_write_char(c, p) bind(C)
    use, intrinsic :: iso_c_binding
    character(C_CHAR), value :: c
    type(C_PTR), value :: p
    <em>...write c...</em>
  end subroutine my_write_char
</pre></div>

<p>Given such a subroutine (along with the corresponding interface definition), you could then export wisdom using:
</p>
<span id="index-c_005ffunloc"></span>
<div class="example">
<pre class="example">  call fftw_export_wisdom(c_funloc(my_write_char), p)
</pre></div>

<span id="index-c_005floc"></span>
<span id="index-c_005ff_005fpointer-4"></span>
<p>The standard <code>c_funloc</code> intrinsic converts a Fortran
<code>bind(C)</code> subroutine into a C function pointer.  The parameter
<code>p</code> is a <code>type(C_PTR)</code> to any arbitrary data that you want
to pass to <code>my_write_char</code> (or <code>C_NULL_PTR</code> if none).  (Note
that you can get a C pointer to Fortran data using the intrinsic
<code>c_loc</code>, and convert it back to a Fortran pointer in
<code>my_write_char</code> using <code>c_f_pointer</code>.)
</p>
<p>Similarly, to use the generic <code>fftw_import_wisdom</code>, you would
define a callback function of the form:
</p>
<span id="index-fftw_005fimport_005fwisdom-1"></span>
<div class="example">
<pre class="example">  integer(C_INT) function my_read_char(p) bind(C)
    use, intrinsic :: iso_c_binding
    type(C_PTR), value :: p
    character :: c
    <em>...read a character c...</em>
    my_read_char = ichar(c, C_INT)
  end function my_read_char

  ....

  integer(C_INT) :: ret
  ret = fftw_import_wisdom(c_funloc(my_read_char), p)
  if (ret .eq. 0) stop 'error importing wisdom'
</pre></div>

<p>Your function can return <code>-1</code> if the end of the input is reached.
Again, <code>p</code> is an arbitrary <code>type(C_PTR</code> that is passed
through to your function.  <code>fftw_import_wisdom</code> returns <code>0</code>
if an error occurred and nonzero otherwise.
</p>
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